Selective separation of uranium from thorium, protactinium and fission products by peroxide dissolution method



2,900,228 Patented Aug. 18, 1959 SELECTIVE SEPARATEUN F r i z: 2 FROMTHORIUM, PRUTAQTINIIUM AND lFliSSION PROD- UCTS BY PEROXEDE DESGLUTIONMETHGD N0 Drawing. Application November 3, 1944 Serial No. 561,836

7 Claims. (Ci. 23-14.5)

The invention relates to the separation of the isotope of uranium havinga mass number of 233 from foreign products, and more particularlyrelates to the separation of such isotope (designated herein as U fromthorium and fission products present in neutron-irradiated thorium.

It is an object of this invention to provide a method of preparing andrecovering U in substantial concentration and further to provide certainnovel and useful compounds of U and Pa Other objects and advantages ofthis invention will become apparent from the accompanying description ofthe invention.

In this specification and claims the name of the ele ment is used todesignate the element generically, either in its elemental state orcombined in a compound unless otherwise indicated by the sense in whichit is used or by a specific designation such as metal or elemental.

It is known that the bombardment of thorium with fast neutrons ofenergies above about 2 million electron volts (2 m.e.v.) results in afission of the thorium.

We have discovered that the bombardment of thorium with neutrons havingenergies of below 1 million electron volts (1 m.e.v.) results not onlyin the production of Pa and ultimately of U through the prolonged decayof Pa 233 and further that U would undergo fission with neutrons of suchlow energies as below 1 million elec- *"on volts (1 m.e.v.) and evenwith thermal neutrons. The production of U is thus complicated by thefact that the U produced by neutron bombardment may be decomposed by thesame bombardment. In accordance with the present invention U inrecoverable concentration may be secured while minimizing the formationof such fission products by proper control of the degree of bombardment.Further we have provided an effective means for recovery of the Uproduced from the unconverted thorium and fission products.

The reaction of thorium with slow and moderately fast neutrons may besummarized as follows:

22.5 min 24.4 days no 5 +9l 92 +5 half-life half-life The fissionproducts which are produced as a result of the fission of U with slowand moderately fast neutrons are, so far as we have been able todetermine, the same as those produced by the fission of U They consistof a large number of elements which generally fall into a light groupwith atomic numbers from 35 to 46 incl. and a heavy group with atomicnumbers from 51 to 60 incl. and which undergo beta decay. The fissionproducts which have a half-life of more than three days will remain inthe reaction mass in substantial quantities at least one month after thetermination of the reaction, and the removal or elimination of theseproducts by our process is particularly advantageous. Among theseproducts are: Sr, Y, Zr, Cb, Ru, Te, 1, Xe, Cs, Ba, La, and

Co of a 20-day half-life, and Ce of a ZOO-day half-life.

In accordance with one embodiment of this invention, a mass of thorium,either metallic thorium or a compound of thorium, such as the oxide,hydrate or carbonate is subjected to the action of neutrons, themajority of which have energies below 1 million electron volts, thereaction of the neutrons with the thorium is terminated prior to thetime when the neutrons are absorbed by the U at the same rate that theyare absorbed by the Th This limit is approximately achieved when theweight ratio of U to unreacted Th is 1 to 100. In other words, thereaction of Th with neutrons should preferably be terminated prior to'when the amount of U is approximately 1 percent of the amount of thoriumpresent in the mass. When the reaction is terminated at or prior to thispoint there is also no danger during the reaction of a substantialdecomposition of the U taking place by a nuclear self-sustaining chainreaction.

It is generally desirable to terminate the reaction of the neutrons WithTh when the amount of U is much less than 1 percent of the unreactedamount of Th in order to reduce the amount of fission products and makeit possible to isolate the U by ordinary chemical means Without the useof large quantities of special materials such as refrigerating devices,radiation shields, special radiation-resistant materials and the like.In order to reduce such special equipment to a minimum and at the sametime have a practical amount of U and Pa 233 for isolation by batchprocess, the reaction is terminated at a ratio of U +Pa to T11 of notless than about 1 to 1 million and frequently between about 1 to 10,000,and l to 1,000.

In order to ensure production of U in a form such as to be recoverablein a concentrated state, the thorium subjected to treatment preferablyis substantially free or contains but negligible amounts of naturaluranium. In any case the amount of natural uranium present should not bein excess of 20 percent by weight of the U produced and production of Usohuld be continued until at least percent by weight of the totaluranium content is U Generally speaking the natural uranium contentshould not exceed about one part by weight per million parts by weightof thorium and preferably should be no more than one part in ten millionparts of thorium.

The reaction of thorimum with neutrons to produce Pa and U may becarried out with neutrons from any suitable neutron source. Where theneutron source provides fast neutrons of above 1 million electron voltsthe fast neutrons are slowed to neutrons having energies of below 1million electron volts by interposing neutronslowing material betweenthe fast neutrons and the thorium. Such neutron-slowing materialsinclude carbon-containing, deuterium-containing, or hydrogen-containingmaterial, for example, graphite, paraffin, and heavy water. Sufficientneutron-slowing material is used so that at least a majority of theneutrons are slowed to energies of below about 1 million electron volts,since at higher energies there is less production of U and considerablefission of the thorium. We may interpose the neutron-slowing materialbetween the fast neutrons and the thorium-containing mass, or we mayadmix neutron-slowing material with the thorium. An intimate mixture ofthorirnum with neutron-slowing material may be readily obtained by usinghydrated thorium compounds, such as Th(OH),,.Xl-I O. Since theslow-neutron-absorption cross section of thorium is some 10 to 40 timeslarger than that of hydrogen, we may suitably use a ratio as high asabout two to four hydrogen atoms per thorium atom without losing anymore than about 10 percent of the neutrons as a result of absorption byhydrogen.

While neutrons obtained from any suitable neutron source may be used, itis desirable to subject the thorium to neutrons from a high intensitysource in order that suitable concentrations of Pa and U may be obtainedin a reasonable length of time.

The thorium is subjected to neutrons from a source of neutrons capableof supplying more than 10 neutrons per second and preferably at least 5l0 neutrons per second to the thorium and in order to secure arelatively high concentration of the U +Pa a mass of thorium weighing nomore than about 20 tons should be used. This body of thorium should besufiiciently thick to absorb at least 50 percent and preferably 75percent or more of the neutrons so supplied. Such high neutron intensitymay be obtained by subjecting thorium to the action of neutrons obtainedby slowing down secondary neutrons obtained from a self-sustaining chainreaction of U U or 94 With neutrons.

By placing the thorium adjacent to a neutron chainreacting masscomprising uranium and/ or 94 in amount sufficient to establish aself-sustaining chain neutron reaction dispersed in a neutron-slowingmeans such as carbon or D 0, neutron itensities of between 5x10 and 10neutrons per second may be supplied to the thorium, and When at least 50to 75 percent of such neutrons are absorbed by the thorium, a ratio of U+Pa to Th of more than 1 to 1 million may be attained in a reasonablelength of time, such as one to three months. In such a case the desireddegree of bombardment may be completed before the preponderant amount ofPa formed has decayed to form U The product obtained by bombardingthorium comprises a preponderant quantity of thorium, a small quantityof U and a smaller amount of fission products. Where the product is notof substantial age, it also contains an appreciable quantity of Pa butthe amount thereof gradually decreases by decay to U Where recovery of amaximum of U is desired, the product is permitted to age until much ofthe Pa has decayed, for example for one or more months. Wherebombardment of the thorium has proceeded over a period of several weeks,an adequate quantity of U may be secured due to decay during thebombardment and further delay to increase the yield may be unnecessary.

We have found that the U may be recovered by means of a soluble peroxidesuch as H 0 Na O etc. The neutron-irradiated thorium containing the U isdissolved in acid, such as an aqueous solution of nitric or hydrochloricacid. It is important that there be present before the precipitation ofthe thorium and uranium from the solution some oxidizing agent, such asnitric acid, so as to be sure that the uranium is in the uranyl form.After the adjustment of the acidity of the solution to a pH of about 3,the thorium and uranium are precipitated together as peroxides by theaddition of hydrogen peroxide in amount in substantial excess of thatrequired to form the corresponding thorium and uranium peroxides. Afterthe removal of the supernatant liquid, the peroxide precipitate isleached or extracted with a suitable alkali-water solution such as anaqueous solution of sodium hydroxide, potassium hydroxide, sodiumperoxide or other alkaline agent capable of extracting the uranium as aperuranate and/ or uranate. Uranium peroxide dissolves readily in sodiumhydroxide or equivalent solution, while thorium peroxide is insoluble,so that a separation of the U from the thorium is effected by thisoperation. The uranium may then be separated from this alkaline solutionby reprecipitation as a peroxide by suitable means, for example byacidifying with an acid such as nitric or hydrochloric acid, so as toprecipitate uranium peroxide. The fission products precipitated with theU at this stage may be substantially completely separated from the U bydissolving the precipitate with nitric acid at a pH of, for example,about 1 and precipitating the {E as the pen oxide with an excess ofhydrogen peroxide, the dissolving and precipitation being repeatedseveral times. Another procedure is to dissolve the precipitatecontaining uranium peroxide and fission products in nitric acid,

.crystallize out the uranyl nitrate hexahydrate by usual used as theneutron-absorbing medium, and after treatment with sodium hydroxide andsodium peroxide solutions so as to extract the U the thorium peroxidewill be ready for use again.

By the above methods of separating U from foreign products we are ableto obtain compositions composed largely or entirely of U compounds,which are substantially free from fission products. The U metal may beproduced from suitable compounds thereof by sodium reduction or any ofthe other known methods for producing uranium metal from compounds ofuranium.

U metal or compounds of U may be shaped into the form of spheres,cylinders, blocks or the like by known methods of shaping uranium metaland compounds. Such shaped articles of manufacture may be used as asource of nuclear power as described in our copending application,Serial No. 565,990, filed November 30, 1944.

While there have been described certain embodiments of our invention, itis to be understood that it is capable of many modifications. Changes,therefore, may be made without departing from the spirit and scope ofthe invention as described in the appended claims, in which it is theintention to claim all novelty in the invention as broadly as possible.

We claim:

1. The method of separating U from foreign products present in neutronirradiated thorium, which method comprises forming a solution of saidneutron irradiated thorium, precipitating the uranium and thorium with asoluble peroxide, treating the peroxide precipitate wtih sodiumhydroxide to selectively dissolve the uramum peroxide, and recovering Ufrom the resulting solution.

2. The method of separating U from thorium, fis

sion products, and protoactinium present in neutron irradiated thorium,which method comprises forming a solution of said neutron irradiatedthorium having a pH of about 3, adding hydrogen peroxide to obtain aprecipitate comprising uranium and thorium, treating the precipitatewith sodium hydroxide to selectively dissolve the uranium, separatingthe dissolved uranium from the thorium, and lowering the alkalinity ofthe solution of said uranium to obtain a precipitate containing U 3. Themethod of separating U from thorium, fission products, and protoactiniumpresent in neutron irradiated thorium, which method comprises forming asolution of said neutron irradiated thorium having a pH of about 3,adding hydrogen peroxide to obtain a precipitate comprising uranium andthorium, treating the precipitate with sodium hydroxide to selectivelydissolve the uranium, separating the dissolved uranium from the thorium,and acidifying the solution to obtain a precipitate of U peroxide.

4. A method of recovering U which comprises forming an aqueous solutionof neutron irradiated thorium containing U precipitating U and thoriumas peroxides and selectively extracting the uranium from the precipitateby means of an aqueous alkaline solution.

5. A method, which comprises irradiating thorium peroxide with neutronsto form Pa which is capable of decay to U extracting the Pa and U fromthe thorium peroxide by means of an aqueous alkali solution andradiating the remaining thorium peroxide with neutrons.

6. The method of separating uranium values from thorium values, whichmethod comprises forming a solution of said thorium and uranium values,precipitating the uranium and thorium values with a soluble peroxide,treating the peroxide precipitate with an aqueous sodium hydroxidesolution to selectively dissolve the uranium peroxide and recoveringuranium values from the resulting solution.

7. A method of separating uranium values from a mixture of thorium,uranium and protactinium values, which method comprises forming asolution of said mixture having a pH of about 3, adding hydrogenperoxide to obtain a precipitate comprising uranium and thorium values,treating the precipitate with sodium hydroxide to selectively dissolvethe uranium values, separating the dissolved uranium values from thoriumvalues, and lowering the alkalinity of the solution of said uraniumvalues to obtain a precipitate containing uranium values.

References Cited in the file of this patent Friend: Textbook ofInorganic Chemistry, vol. VII, part III, p. 311 (1926); published byCharles Griffin & Co., Ltd., London.

Rosenheim et 211.: Chemical Abstracts, vol. 23, p. 4634 (1929).

Meitner et al.: Chemical Abstracts, vol. 32, p. 7815 (1938).

Hopkins: Chapters in the Chemistry of the Less Familiar Elements, vol.II, chapter 18, Uranium, pp. 7, 14; Stripes Publishing Co., Champaign,Ill. (1939).

Hahn et a1.: Chemical Abstracts, vol. 36, p. 6893 (1942).

1. THE METHOD OF SEPARATING U233 FROM FOREIGN PRODUCTS PRESENT INNEUTRON IRRADIATED THORIUM, WHICH METHOD COMPRISES FORMING A SOLUTION OFSAID NEUTRON IRRADIATED THORIUM, PRECIPITATING THE URANIUM AND THORIUMWITH A SOLUBLE PEROXIDE, TREATING THE PEROXIDE PRECIPITATE WITH SODIUMHYDROXIDE TO SELECTIVELY DISSOLVE THE URANIUM PEROXIDE, AND RECOVERYU233 FROM THE RESULTING SOLUTION.